1. Field of the Invention
The present invention relates to a phase shifter for performing orthogonal modulation/demodulation in digital communications and, more particularly, to a phase shifter using an RC filter composed of a resistive element and a capacitive element.
2. Description of the Prior Art
In digital communications, an orthogonal modulation/demodulation technology plays an important role. In orthogonal modulation/demodulation, two orthogonal local signals or high-frequency (RF) signals are used. The phase shifter is a circuit for generating an orthogonal signal, i.e., a signal having a phase difference of 90xc2x0. The phase shifter must have the following characteristics: (1) small phase and amplitude errors, (2) broad in band, and (3) low in power consumption.
Phase shifters are usually classified as (A) a type using an RC filter composed of a resistive element and a capacitive element, and (B) a type using a frequency divider compose of a flip-flop. The phase shifter of the type (B) has an advantage of small phase and amplitude errors in a broad band. In the phase shifter of the type (B), however, there are fundamental constraints imposed. That is, because of a necessity of a reference signal having a frequency higher by frequency division ratio times, an upper limit of an operation frequency is restricted by an operating limit of the flip-flop.
On the other hand, in the case of the phase shifter of the type (A), because of a fixed RC time constant in an integrated circuit, an amplitude error is increased only with a simple combination of a low pass filter and a high pass filter, and thus a correction circuit such as PLL must be added. A phase shifter using an all pass filter is also available. However, this phase shifter was impractical, because a ratio of elements necessary for achieving an RC constant made manufacturing of an integrated circuit difficult in most cases. In addition, when a frequency becomes higher, an RC time constant must be reduced. However, there is a limit to a capacity value for accurate formation in the integrated circuit. Thus, a value of resistance must be reduced, making it necessary to lower input impedance of the RC filter. For driving the RC filer, a current proportional to an inverse number of impedance of the RC filter is necessary. Consequently, as a frequency is higher, consumption of current is increased.
As an RC filter capable of obtaining an orthogonal signal broad in a band, and small in both amplitude and phase errors, a polyphase filter having resistive elements and capacitive elements cyclically connected is known. However, the polyphase filter is a 4-phase RC filter, where input impedance is smaller than that of a normal single phase RC filter. Thus, the polyphase filter must be used in small amplitude, or by increasing a driving current.
FIG. 1 is a block diagram of a conventional phase shifter described in Japanese Patent Application Laid-Open No. 11 (1999)-298293. This conventional phase shifter includes a differential amplifier 101 for driving, cascade-connected RC polyphase filters (simply referred to as polyphase filters, hereinafter) of N stages 201, . . . , 20N, and differential amplifiers 301, . . . , 30N used as buffer circuits. In this conventional phase shifter, the polyphase filter 201 is driven by the differential amplifier 101. However, since a large driving current cannot be supplied for the above-described reason, driving must be carried out by small amplitude. In addition, in order to achieve a broader band, a plurality of polyphase filters are cascade-connected. However, signal voltages are decayed at the polyphase filters 201, . . . , 20N. Thus, the differential amplifiers 301, . . . , 30N are provided between the polyphase filters to amplify the signal voltages. Because of such configuration, a circuit size was increased, creating a problem of an increase in power consumption. Especially, in a device driven by a battery such as a portable equipment, an increase in power consumption is not preferable, because it directly shortens use time.
The conventional phase shifter using the polyphase filters is advantageous in that a broad band can be achieved, and errors in both amplitude and phase can be suppressed. However, the necessity of providing the buffer differential amplifiers for driving with small amplitude has increased the circuit size, consequently increasing the power consumption.
The present invention is devised in view of such a circumstance, and an object of the invention is to provide a phase shifter improved for a reduction in power consumption, inheriting the advantages of the phase shifter using the polyphase filters, i.e., the capabilities of achieving a broad band, and suppressing errors in both amplitude and phase.
In order to achieve the above-described object, in accordance with an aspect of the present invention, there is provided a phase shifter, comprising: a voltage-to-current conversion circuit for receiving an input signal, converting a voltage value of the input signal into a current value, and outputting an input current signal; a polyphase filter composed of a plurality of cascade-connected stages of RC filters for receiving the input current signal, and outputting a polyphase phase-shifted current signal, each RC filter having resistive and capacitive elements connected alternately cyclically, one end of each resistive element being set an input terminal, and the other end of each resistive element being set as an output terminal; and a load circuit including a current-to-voltage conversion circuit for converting each current value of the polyphase phase-shifted current signal into a voltage value, and outputting a output signal. Since the voltage value of the input signal is converted into the current value, and supplied to the polyphase filter, the number of buffer circuits conventionally necessary for amplifying signals can be reduced, and consumption of power can be reduced.
According to the phase shifter of the invention, the voltage-to-current conversion circuit includes a differential amplifier composed of a first transistor having an emitter connected to a constant current source, one input signal as a balanced signal entered to a base, and a collector connected to one input terminal of the polyphase filter, and a second transistor having an emitter connected to the constant current source, the other input signal connected to a base, and a collector connected to the other input terminal of the polyphase filter, and a current signal corresponding to a voltage value of the input signal is supplied to the polyphase filter.
According to the phase shifter of the invention, the polyphase filter is constructed by cascade-connecting n stages (nxe2x89xa72) of RC filters, each RC filter including a first resistor having one end connected to a first input terminal, and the other end connected to a first output terminal, a second resistor having one end connected to a second input terminal, and the other end connected to a second output terminal, a third resistor having one end connected to a third input terminal, and the other end connected to a third output terminal, a fourth resistor having one end connected to a fourth input terminal, and the other end connected to a fourth output terminal, a first capacitor having one end connected to the other end of the first resistor, and the other end connected to one end of the second resistor, a second capacitor having one end connected to the other end of the second resistor, and the other end connected to one end of the third resistor, a third capacitor having one end connected to the other end of the third resistor, and the other end connected to one end of the fourth resistor, and a fourth capacitor having one end connected to the other end of the fourth resistor, and the other end connected to one end of the first resistor, the first and second input terminals of the RC filer of the first stage in an input side are connected in common to enter one input current signal, the third and fourth input terminals are connected in common to enter the other input current signal, and first to fourth phase-shifted output current signals are outputted from the first to fourth output terminals of the RC filter of the n-th stage in an output side. Since the plurality of stages of RC filters are cascade-connected, a phase-shifting operation can be performed in a broader band.
According to the phase shifter of the invention, the load circuit includes first to fourth transistors having emitters respectively connected to the first to the fourth output terminals of the polyphase filter for outputting a 4-phase signal as the polyphase phase-shifted current signal, bases connected in common to a bias power source, and operated as current-to-voltage conversion circuits each for converting an input current value of the polyphase phase-shifted current signal into a voltage value, first to fourth load resistive elements having ends connected to collectors of the first to fourth transistors, and the other ends connected to a power source, a connection point between the first resistive element and the collector of the first transistor is set as an output terminal of an output signal of 0xc2x0, a connection point between the second resistive element and the collector of the second transistor is set as an output terminal of an output signal of 90xc2x0, a connection point between the third resistive element and the collector of the third transistor is set as an output terminal of an output signal of 180xc2x0, and a connection point between the fourth resistive element and the collector of the fourth transistor is set as an output terminal of an output signal of 270xc2x0. As output signals, signals phase-shifted by 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0 can be obtained.
According to the phase shifter of the invention, the load circuit includes first and second transistors having emitters connected in common to the first output terminal of the polyphase filter for outputting a 4-phase signal as the polyphase phase-shifted current signal from each of the first to fourth output terminals, and constituting a first differential circuit, third and fourth transistors having emitters connected in common to the second output terminal of the polyphase filter, and constituting a second differential circuit, fifth and sixth transistors having emitters connected in common to the third output terminal of the polyphase filter, and constituting a third differential circuit, seventh and eighth transistors having emitters connected in common to the fourth output terminal of the polyphase filter, and constituting a fourth differential circuit, and first to fourth resistive elements having ends connected to a power source, bases of the first, third, fifth and seventh transistors are connected in common to be set as a first local signal input terminal, bases of the second, fourth, sixth and eighth transistors are connected in common to be set as a second local signal input terminal, a balanced local signal is supplied from the outside to the first local signal input terminal, a balanced local signal of opposite sign is supplied to the second local signal input terminal, a connection point, to which the other end of the first resistor and collectors of the first and fifth transistors are connected in common, is set as one output terminal of a in-phase output signal, a connection point, to which the other end of the second resistor and collectors of the second and sixth transistors are connected in common, is set as the other output terminal of the in-phase output signal, a connection point, to which the other end of the third resistor and collectors of the third and seventh transistors are connected in common, is set as one output terminal of an orthogonal-phase output signal, and a connection point, to which the other end of the fourth resistor and collectors of the fourth and eighth transistors are connected in common, is set as the other output terminal of the orthogonal-phase output signal. By entering an orthogonally modulated signal to the phase shifter, it is possible to perform orthogonal demodulation.
According to the phase shifter of the invention, the load circuit includes first and second transistors having emitters connected in common to the first output terminal of the polyphase filter for outputting a 4-phase signal as the polyphase phase-shifted current signal from each of the first to fourth output terminals, and constituting a first differential circuit, third and fourth transistors having emitters connected in common to the second output terminal of the polyphase filter, and constituting a second differential circuit, fifth and sixth transistors having emitters connected in common to the third output terminal of the polyphase filter, and constituting a third differential circuit, seventh and eighth transistors having emitters connected in common to the fourth output terminal of the polyphase filter, and constituting a fourth differential circuit, and first and second resistive elements having ends connected to a power source, bases of the first and sixth transistors are connected in common to be set as a first in-phase local signal input terminal, bases of the second and fifth transistors are connected in common to be set as a second in-phase local signal input terminal, bases of the third and eighth transistors are connected in common to be set as a first orthogonal-phase local signal input terminal, bases of the fourth and seventh transistors are connected in common to be set as a second orthogonal-phase local signal input terminal, a balanced in-phase local signal is supplied from the outside to the first in-phase local signal input terminal, a balanced in-phase local signal of opposite sign is supplied to the second in-phase local signal input terminal, a balanced orthogonal-phase local signal is supplied from the outside to the first orthogonal-phase local signal input terminal, a balanced orthogonal-phase local signal of opposite sign is supplied to the second orthogonal-phase local signal input terminal, a connection point, to which the other end of the first resistor and collectors of the first, third, fifth and seventh transistors are connected in common, is set as one output terminal of an output signal, and a connection point, to which the other end of the second resistor and collectors of the second, fourth, sixth and eighth transistors are connected in common, is set as the other output terminal of the output signal. This phase shifter can perform an image rejecting operation.
According to the phase shifter of the invention, the load circuit includes first and second transistors having emitters connected in common to the first output terminal of the polyphase filter for outputting a 4-phase signal as the polyphase phase-shifted current signal from each of the first to fourth output terminals, and constituting a first differential circuit, third and fourth transistors having emitters connected in common to the second output terminal of the polyphase filter, and constituting a second differential circuit, fifth and sixth transistors having emitters connected in common to the third output terminal of the polyphase filter, and constituting a third differential circuit, seventh and eighth transistors having emitters connected in common to the fourth output terminal of the polyphase filter, and constituting a fourth differential circuit, and first to fourth resistive elements having ends connected to a power source, bases of the first to eighth transistors are connected in common to a bias power source, a connection point, to which the other end of the first resistive element and collectors of the first and fourth transistors are connected in common, is set as one output terminal of an output signal phase shifted by 0xc2x0, a connection point, to which the other end of the second resistive element and collectors of the third and sixth transistors are connected in common, is set as an output terminal of an output signal phase-shifted by 90xc2x0, a connection point, to which the other end of the third resistive element and collectors of the fifth and eighth transistors are connected in common, is set as an output terminal of an output signal phase-shifted by 180xc2x0, and a connection point, to which the other end of the fourth resistive element and collectors of the second and seventh transistors are connected in common, is set as an output terminal of an output signal phase-shifted by 270xc2x0. This phase shifter can accurately shift phases in a very wide range when used in combination with an amplitude equalizer.
The foregoing, and other related objects and features become apparent upon reading of new items specified in the description made with reference to the accompanying drawings, and appended claims.